This invention relates to distributed feedback (DFB) lasers. DFB lasers with nominally identical gratings and waveguides that are derived from the same wafer can typically be expected to have a device to device wavelength scatter of up to about 1 nm. A tuning range of 1 nm is therefore required so that all devices from one wafer can be matched to a fixed local oscillator frequency. When wafer-to-wafer variations in wavelength are considered, a greater tuning range is desirable, preferably about 5 nm, though for a number of applications about 2 nm should suffice.
One method of tuning the output of a DFB laser is described in European Patent Application Specification No. EP 0 169 567 A2 to which attention is directed. The method described therein involves a construction of laser chip in which distributed feedback is provided along only a portion of the whole length of the optical cavity within the chip, that portion extending from one end. At the other end a highly reflecting facet is provided. The emission wavelength is tuneable over a restricted range by adjusting the current drive applied to the region of the optical cavity extending between the highly reflecting facet and the distributed feedback region. Adjustment of this drive affects the refractive index, thereby affecting the phase of light reflected back into the distributed feedback region, and hence the frequency of laser emission.